Description:
------------
---
From manual page: http://www.php.net/datetime.sub#refsect1-datetime.sub-
description
---
When you have a datetime object you can modify the date adding or subtracting
month. If you do that, the result is always unpredictable, because the mktime,
the sub, or the modify function, always subtract the amount of days of the
previous month. I'll make an example:
If I do
<?php
$date = new DateTime('2001-03-31');
$interval = new DateInterval('P1M');
$date->sub($interval);
echo $date->format('Y-m-d');
?>
I obtain 2001-03-03 as result, but I've expected to have 2001-02-28, because the
previous month is february, not 28 days first.
We have the same problem if we add some months to our starting date:
<?php
$date = new DateTime('2001-03-31');
$interval = new DateInterval('P1M');
$date->add($interval);
echo $date->format('Y-m-d');
?>
I obtain 2001-05-01 as result, but also in this case I've aspected a different
result, i think the right value is 2001-04-30, because you have to add the days
of the next month to obtain the right result, not the days of current month.
.
The problem is also the communication with mySql, because mySql calculate the
right result, so the comparison between two date it's impossible
Test script:
---------------
<?php
$date = new DateTime('2001-03-31');
$interval = new DateInterval('P1M');
$date->sub($interval);
echo $date->format('Y-m-d');
?>
Expected result:
----------------
2001-02-28
Actual result:
--------------
2001-03-03

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php
This is expected behaviour. +1 month and -1 month add or remove "1" from the month number. The dates look odd because the day count under or overflows.